And who wants to carry an external HDD with you if you could put a second HDD inside?
Does Apple include a second HDD with every notebook? No. But almost everybody needs a second HDD for backups. Only if you do online backup only, you do not need a second HDD. A
Does Apple include an ODD with every notebook? No. Not with Air, not with some models of the future MBP. Does everybody need an ODD? If you watch DVDs only on your home entertainment center, download all your software and music, you do not need an ODD. A significant number of people will need it but 'almost everybody'? No.
In short, more people need a second HDD than need an ODD. Yet Apple ships almost computers without a second HDD and only a small portion without an ODD.